How does conscious experience arise from a kilogram of damp, grey, rumpled tissue? There is, of course, no easy answer.  Consciousness tends to make scientists nervous - even the ones who have pitched their careers towards unravelling the billions of cells and thousands of miles of fibres that make it possible.  The field of neuroscience employs an army of researchers, nearly all chiselling slavishly away at miniscule details of how the brain functions.  Precious few will announce “consciousness” as the target of their research, for fear of becoming a laughing stock.  The last thirty years has seen no shortage of treatises on the subject, but these have mostly been written by philosophers, or by Nobel-winning scientists tending towards philosophy in their old age (see Francis Crick or Gerry Edelman).  Nevertheless, scientists today can examine human brain activity more precisely than ever before and plenty of ongoing research, including the two examples discussed below, directly or indirectly concerns this contentious phenomenon.  So why not shoot for the biggest teddy in the sideshow?

The problem is partly linguistic, and partly historical.  “Consciousness” is an intimidating, all-encompassing term that arguably has no place at all in scientific discourse.  It can refer to many different things: awareness, wakefulness, experience, self-perception, thought… In fact, it probably amounts to a delicate assembly of several processes, such as our ability to perceive the world and to lay down memories of those perceptions.  As we will see, modern science prefers to consider such components individually, but the study of consciousness as an entity is even older than the scientific method.  As a great, inward mystery, it has occupied philosophers since they first contemplated their own existence.  In the 17th century Descartes articulated the problem as one of reconciling mind and matter: how do thought and brain mix? Where is the intersection? He famously decided it was in the pineal gland, a tiny nub near the centre of the brain which in fact secretes melatonin.  Misplacement of the soul aside, Descartes’ “dualism” dominated thinking on the subject for hundreds of years and it is still the way many of us conceptualise the problem today.  Biology was much slower to weigh in; over subsequent centuries, knowledge of the brain’s anatomy steadily accumulated but the whole enterprise was somewhat derailed by the rise of phrenology in the early 1800s.  The deluded confidence with which phrenologists ascribed mental faculties to lumps and bumps in the skull remained disastrously high for most of that century and it was not until the turn of the next one that a new age of science, spearheaded by two completely new disciplines, promised real progress. 

In the 1890s, William James founded a new science with the question of consciousness at its core.  Indeed, for much of his two-volume opus The Principles of Psychology, James prattled on about it in a very natural and intuitive way.  He saw it as a fluid sequence of thoughts and perceptions, for which he actually coined the term “stream of consciousness”.  The best way to study this stream was Introspective Observation: “looking into our own minds and reporting what we there discover”.  So-called “introspectionists”, particularly from two laboratories in Leipzig and Cornell, spent years quizzing carefully trained experimental subjects about the contents of their own minds, trying to assemble a periodic table of perception.  Unfortunately their painstaking labour was hamstrung because the data were impossible to standardise; the Cornell camp ended up with a grand total of 44,000 discernible sensations while Team Leipzig managed only 12,000.  In between the wars, a new school of psychology tried to secure more credibility by restricting its study to observable behaviour.  The “behaviourists” dealt in input and output, stimulus and response, and basically ignored the possibility that anything interesting might happen in between.  Things improved with the development of cognitive psychology in the 1960s, but only insofar as “consciousness” reappeared as a box in a flow chart.  The cognitivists treated the brain like a computer, and made good use of new ideas about information processing.  Overall however, psychology in any of its forms has never managed to firmly fasten its hypotheses about consciousness to the hardware of the brain.

What about neuroscience? This was the other horizon-shifting success of the late 19th century: experiments that described the brain’s electrical currents and, crucially, its constituent brain cells.  Camillo Golgi and Santiago Ramón y Cajal, ferocious intellects and fierce rivals, together accomplished the latter and had to share (tersely) the 1906 Nobel Prize.  Their trailblazing microscopic images, still remarkable to behold, presented the neuron to the world.  Here at last was a plausible building block for the nervous system - intricate and varied in structure, and capable of receiving and forwarding electrical signals.  This discovery changed the game forever, and for the branch of science that began here with the “neuron doctrine” and soon co-opted its name, the 20th century was one of almost indescribable progress.  Neuroscientists elucidated countless details of how neurons transmit and aggregate signals, how they receive sensory stimuli and effect responses.  But despite all these advances at the level of neurons themselves, or perhaps because of them, the question of consciousness seemed distant.  It acquired the aura of the unassailable, and the business of relating it to brain cells firing was done speculatively and mostly in the twilight of distinguished careers.  After years publishing “serious” science in top journals, it might be said of any successful neuroscientist, especially one with a propensity for pontificating, “Oh, isn’t it time for them to write a book on consciousness?” Such cynical murmurs are perhaps unfair; many such books exist and they are not all without merit.  The fact remains, however, that most working neuroscientists have shied away from the subject.

And yet today, we have the gadgetry to look inside the active - conscious - human brain and detect signals that relate very closely to the firing of neurons.  These expensive toys are the stuff of William James and Camillo Golgi’s wildest dreams, and they bring psychology and neuroscience into the same room.  Into the same enormous tubular magnet, in fact, for the technique in question is fMRI, or functional magnetic resonance imaging.  In a nutshell, fMRI uses a really, really strong magnetic field to detect tiny changes in blood flow within the brain; active brain areas receive extra blood and active brain cells suck more oxygen out of it.  A computer records these changes as tiny “voxels” of activity in a detailed 3D model of the brain, while that brain’s owner lies patiently inside the fMRI scanner, trying not to move.  Since the early 1990s, scientists have been harvesting these flickering populations of pixels and gaining glimmers of insight into every imaginable aspect of human brain function - including consciousness.  Mind you, they are still reticent to aim straight for the biggest prize; a giant teddy bear is, after all, awkward to carry home.  Scientists thrive on details, and will nearly always tackle big problems by breaking them down into small pieces.  This approach, coupled to the power of functional brain imaging, is beginning to bear fruit.  There is even a strong case to be made, by scientists and philosophers alike, that once we understand the component processes that together might be called “consciousness”, there is nothing further to explain.  I would like to leave that particular question in the “too hard” basket (philosophers even call it “The Hard Problem“ - there are books on it) and instead simply offer a sample of current research.  I have plucked two examples directly from the journal pages of early 2010: in one, subjects stare at invisible houses; in the other, a vegetative patient answers questions using only his brain.

Perception is one component of consciousness that is particularly accessible to experiments.  If we are conscious of a stimulus, what difference does this make to the neuronal activity that it stimulates? It may seem paradoxical that your brain processes stimuli of which you are unaware, but this is precisely the circumstance explored by Aaron Schurger and his colleagues at Princeton.  Writing in Science in January, Dr Schurger reported experiments in which volunteers lay in an fMRI scanner looking at countless pictures of faces and houses, some of which they couldn’t see.  These “invisible stimuli” are created by presenting simplified monochrome illustrations separately to each eye; if the foreground and background colours in one image are reversed, the image disappears.  Dr Schurger’s subjects were asked to state whether the drawing before them was a face or a house - an easy task, until an example with reversed colours was presented.  In these cases, the subjects’ brains still showed activity consistent with the image being processed, even though they couldn’t perceive it.  The researchers looked in the visual cortex for differences separating the activity in these trials from those in which the same image was consciously perceived.  Interestingly, they found that the activity during conscious perception was more reproducible than during invisible stimuli: the spatial pattern of active voxels was similar from trial to trial.  It has already been proposed that what designates a particular pattern of brain activity “conscious” might be its intensity (how many neurons are firing, and how many times) or its synchronicity (the degree to which the firing events are coordinated); Dr Schurger’s results suggest that consistency is also key. 

This reproducibility, however, can only be observed across multiple presentations of the same stimulus; it cannot be the decisive factor that the brain uses to make a perception “conscious”, because we don’t need multiple presentations in order to see something.  These comparisons are a privilege of experiments like Dr Schurger’s.  They inform us about the nature of the processes themselves - in this case, perhaps the neuronal activity behind a conscious visual perception is more consistent or stable because it is fine-tuned by connections from other brain areas.  (This is beyond the reach of Dr Schurger’s data, but it is a reasonable hypothesis.) These other linked areas might be responsible for other facets of consciousness.  Importantly, the connections are likely to act in both directions, so that visual perceptions influence other processes, like attention or decision-making, but are also constrained by them.  “Recurrent connectivity” that goes back on its tracks like this is a common feature in the literature of consciousness.  Gaining access to various parts of the brain, and receiving input from them in return, is likely to be crucial for rendering information conscious. 

A more fundamental distinction is between consciousness and unconsciousness.  In this case we are concerned with consciousness as “wakefulness”, an aspect that is also open to investigation with fMRI.  It is particularly important in the clinical setting, where families long to communicate with loved ones devastated by brain injury, and where the decision to provide or remove life support can rest on a definition or a diagnosis.  In a fascinating series of experiments over several years, scientists from Cambridge and Liege have performed fMRI on comatose patients and found that some patients classified as “vegetative” can in fact alter their brain activity on command.  Four years ago, Adrian Owen and his collaborators identified such a patient and found that if she was instructed to imagine playing tennis or moving around her own house, areas of the brain specific to each task would light up.  A new study from the same team, published last month in the New England Journal of Medicine, describes the proportion of minimally responsive patients in whom such function might be uncovered (specifically, 5 out of 54).  These numbers are not high, although they do make a compelling case for more careful diagnosis.  Moreover, the fact that activity in specific regions is evoked following specific instructions does not mean that sundry other elements of consciousness, such as self-awareness and memory, are present.  To trivialise the whole affair - and to spin Descartes in his grave - it will never be as simple as “I think I’m playing tennis, therefore I am.”

In a few special cases, however, it might come close.  Dr Owen’s paper has an intriguing addendum, in which one patient was instructed to answer yes-or-no questions by switching between spatial thoughts (navigation) and procedural ones (tennis).  Remarkably, he “answered” correctly when questioned about the names of his family.  This is a breathtaking finding, tinged with sadness; it offers the possibility of communicating with outwardly unresponsive patients, but it requires one of the most expensive and computationally demanding techniques in neuroscience.  Bringing this sort of communication to the bedside is an enormous challenge but Dr Owen and others are already working on the possibility of using electroencephalography (EEG), which requires electrodes taped to the scalp instead of a ten-tonne, multi-million-dollar magnet.  Years of research and myriad ethical considerations remain to be negotiated, but the potential benefits for patients and their families are immense.

These two recent papers approach consciousness from different angles, as well as from different sides of the Atlantic.  They illustrate the slow but steady progress that science is now making on the subject.  This is quite a contrast to the state of affairs just over a century ago, when the first psychologists and neuroscientists were poised to storm nature’s “last citadel”.  Perhaps partly because they still saw it as such an ultimate, singular challenge, their advances across its walls were slow or stalled for many years.  Now, it is a routine subject for experiment.  The boundary between psychology and neuroscience is blurred by techniques like fMRI and for researchers chipping away in the field, consciousness is not the intimidating edifice it once was.  It is an observable phenomenon, a characteristic of the pixels on the screen.  Scientists are reticent to talk about it in the same terms that made it an inscrutable apparition, because they view it as an assembly of complex - but comprehensible - processes.  By understanding its components, we may explain it as a whole, or perhaps come to appreciate that it is simply the sum of its remarkable parts.  We can also start to understand the consequences when this physical process, the flood of electrochemical activity that makes us who we are, is damaged.  And then, if there is anybody in there, we might just be able to say hello.

The Thought Police by James Randell